This application claims the benefit of Japanese patent application No. 10-209709, filed Jul. 24, 1998, which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a receiver with a search circuit for detecting stations, and more particularly, to a receiver with a circuit for controlling the sensitivity of the search circuit.
2. Description of the Related Art
A conventional receiver, such as an AM or a FM receiver, usually contains an Auto Gain Control (AGC) circuit. An AGC adjusts the output signal level of an amplifier to a specific level when the amplifier receives a strong input signal level.
Such a receiver also has a search circuit with a station detector for detecting stations. When a user activates auto search, the search circuit compares the reception level to a reference level set in the receiver. If the reception level is higher than the reference level, the receiver picks up the frequency corresponding to the reception level. On the other hand, if the reception level is lower than the reference level, the receiver does not pick up anything. A specific field strength is usually used as a reference level. Therefore, a search circuit compares the specific field strength corresponding to a reference level to a received field strength level. However, this conventional receiver has a problem known as a xe2x80x9cmiss-stop.xe2x80x9d The miss-stop occurs when the conventional receiver does not detect a desired station during an auto search operation although a station exists and has enough input signal level being supplied through the antenna.
The xe2x80x9cmiss-stopxe2x80x9d occurs as a result of the AGC circuit. The Radio Frequency Auto Gain Control (RF-AGC) circuit generates a control signal to a Radio Frequency Amplifier (RF-AMP) based on a signal level corresponding to the received station""s level. This operation causes a decrease in the output signal level of the RF-AMP. In other words, the above operation decreases the field strength level corresponding to the current frequency and the field strength levels corresponding to other frequencies received by the antenna. As a result, the search circuit cannot detect the current frequency because the field strength level is higher than the received field strength level. For instance, a xe2x80x9cmiss-stopxe2x80x9d occurs when a receiver is near a strong signal level and the antenna receives the signal. The signal is then amplified by a Radio Frequency Amplifier (RF-AMP) based on the control signal from the RF-AGC and results in a low gain. Thus, the receiver cannot detect the station corresponding to the current field strength level because of the decrease in the gain caused by the RF-AGC.
Japanese laid-open publication number Hei 6-13921 suggested an improvement to the xe2x80x9cmiss-stopxe2x80x9d problem. In a receiver based on the Hei 6-13921, when the auto search operation with a receiver (an AM receiver) is started, the receiver decreases the sensitivity of the RF-AGC circuit as compared to a normal operation. As a result, a gain of the RF-AMP during the auto search operation is decreased as compared with the gain in the normal operation. So, it is possible for the station detector to properly detect stations in a strong reception level area. However, this type of receiver also has a problem caused by changing the sensitivity of the RF-AGC circuit and results in another type of xe2x80x9cmiss-stop,xe2x80x9d where a receiver picks up a station even though no such station exists.
In a receiver based on the Hei 6-13921, the xe2x80x9cmiss-stopxe2x80x9d occurs because the RF-AMP uses a transistor as an amplifier device. A transistor as an amplifier device makes harmonics based on non-liner characteristics, such as square characteristics, when the transistor receives a strong signal level. This results in the second or the third harmonics being detected as a station. Thus, when one of the harmonics is higher than the reference level set in the receiver, the search circuit detects it as a real station.
Accordingly, the present invention is directed to a receiver with a search circuit that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to detect a station correctly during a search operation without being influenced by any reception conditions.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.
To achieve the objects and in accordance with the purpose of the invention, as embodied and broadly described herein, the receiver includes a RF-AGC circuit for gain control of a RF-AMP and for generating a control signal according to an output signal of the RF-AMP; a search circuit including a reception condition detector for generating the reception condition signal based on the output signal of a RF-AMP; a sensitivity change circuit for receiving the control signal from a RF-AGC Circuit and for changing the sensitivity of the search circuit so as to compensate a loss of the reception condition signal caused by the RF-AGC circuit; and a station detector for outputting a station detection signal based on a reception condition signal and reference signal.
In another aspect, the receiver includes a RF-AGC circuit, which outputs a first control signal, corresponding to an output signal of an RF-AMP, to control the output of the RF-AMP; and a second control signal, corresponding to an output signal of the RF-AMP, to a sensitivity change circuit; a search circuit including a reception condition detector for generating a reception condition signal according to the output signals of the RF-AMP; a sensitivity change circuit for adjusting the sensitivity of the search circuit according to the second control signal so that the loss in the reception condition signal caused by the RF-AGC circuit is compensated; and a station detector for generating a station detection signal based on a reception condition signal and a reference signal.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.